The invention relates to an integrated optical wavelength multiplexer and demultiplexer for optical monomode glass fiber transmission systems and its use.
The full utilization of the transmission capacity of glass fiber transmission systems requires optical wavelength multiplexing and demultiplexing methods, briefly termed WDM (wavelength division multiplexing). In WDM, several different optical frequencies form several information channels which can be transmitted simultaneously without interference through one glass fiber.
For monomode optical transmission, WDM methods use micro-optical elements. (See, e.g. T. Uchida, et al. "Micro-Optic Devices For Optical Communications. 4th European Conference on Optical Communication, Geneva 1978, Contribution 8-1, pages 374-382 G. Winzer, et al. "Single-mode and multimode all-fiber directional couplers for WDM." Appl. Opt., Vol. 20, No. 18, pages 3128-3135 (1981); M. Seki, et al "20-Channel Micro-Optic Grating Demultiplexer For 1.1-1.6 .mu.m Band Using a Small Focusing Parameter Graded-Index Rod Lens." Electron Lett., Vol. 18, No. 6, pages 257-258 (1982).) Alternatively, monomode WDM uses fiber optical elements. (See, e.g. M. Digonnet, et al. "Wavelength multiplexing in single-mode fiber couplers." Applied Optics, Vol. 22, No. 3, pages 484-491 (1983); S. K. Sheem, et al. "Single-mode fiber wavelength multiplexer." J. Appl. Phys., Vol. 51, No. 8, pages 4050-4052 (1980).) All these methods involve a comparatively great effort for the manufacture and the incorporation of the wavelength-selective elements and for the adjustment of the individual monomode fibers.
Because of these disadvantages, WDM devices have been examined in which integrated optical elements are used. The integrated optics permit manufacturing many wavelength-selective elements with the required monomode strip waveguides on one substrate, and moreover yield the advantages of manufacture according to planar technology. For WDM methods, especially two effects have been utilized by integrated optics: the wavelength-selectivity of the directional coupler with non-identical waveguides (R. C. Alferness, et al. "Tunable optical waveguide dirctional coupler filter." Appl. Phys. Lett, Vol. 33, No. 2, pages 161-163 (1978) and the great wavelength dependence of the TE-to-TM conversion (R. C. Alferness, et al. "Electro-optic waveguide TE-TM mode converter with low drive voltage." Opt. Lett., Vol. 5, No. 11, pages 473-475 (1980)).
The disadvantages of the directional coupler reside on the one hand in the non-identical strip waveguides, which lead to different coupling efficiencies when coupling to glass fibers or suceeding cascade-connected directional couplers, and on the other hand in the necessity of dimensioning differently all the individual directional couplers in the case of a multi-channel WDM. Moreover, the large length of the directional coupler (.about.1 cm) and the low cross-talk damping obtained (-17 dB) militate against a cascade arrangement of several directional couplers for the use in a multichannel WDM.
The TE-to-TM converter has a shorter length (6 mm) and a higher cross-talk damping (20 dB), but it has the great disadvantage that it comprises only one strip waveguide and consequently does not permit either a spatial joining or a spatial separation of the spectrally decomposed signals. Thus, it is not suitable for multichannel WDM use or is only limited to the demultiplexer side.